1. Field of the Invention
The present invention relates to an image pickup apparatus, and more particularly, to an image pickup apparatus including an image pickup lens for forming a subject image on an image pickup element such as a CCD sensor and a CMOS sensor. The present invention relates to an image pickup apparatus including an image pickup lens, which is suitable for a monitoring camera, a network camera, an on-board camera, a digital camera, and the like.
2. Description of the Related Art
Monitoring cameras have hitherto been required to be compact and lightweight with a wide imaging range and be compatible with low illuminance. With recent improvements in resolution of sensors, the monitoring cameras are now required to have high resolution compatible with full HD. Because of this, an image pickup lens to be mounted in a monitoring camera is required to be compact and lightweight with a wide horizontal angle of field of 75° or more and a small f-number and have high optical performance. Japanese Patent Application Laid-Open No. 2005-221920 proposes a low-cost and compact optical system having satisfactory optical performance with a maximum angle of field of approximately 85°. Japanese Patent Application Laid-Open No. 2008-233610 proposes an optical system having satisfactory optical performance with a maximum angle of field of 70° to 80°, an f-number of 2.0, and six lenses.
In general, an image pickup lens for monitoring is preferred to have satisfactorily corrected chromatic aberration in order to obtain satisfactory imaging performance.
In general, the monitoring cameras are used in a wide temperature range including cold areas and tropical areas. It is therefore important particularly for a so-called pan-focus lens having no focus mechanism to suppress a temperature-related imaging position change because the occurrence of the temperature-related imaging position change may change the range of the depth of field in focus (that is, the pan-focus range).
Japanese Patent Application Laid-Open No. 2005-221920 uses a plastic aspherical lens, which has a large temperature-related refractive index change, in order to obtain satisfactory optical performance with a compact and lightweight structure at low cost. Accordingly, there is a problem in that an imaging position may be changed due to a temperature change.
In Japanese Patent Application Laid-Open No. 2008-233610, the structure in which a third lens unit contributing to image formation is made of a glass material cannot satisfactorily correct the secondary spectrum of axial chromatic aberration.